1. Field of the Invention
The invention relates to corrosion and chip resistant coating compositions that can be used for highly stressed steel such as automotive springs, and to highly stressed steel coated with the coating compositions.
2. Description of the Related Art
Compositions for coating steel are generally well known in the art. U.S. Pat. No. 5,334,631 discloses a coating composition comprising an epoxy resin, a curing agent, lamellar zinc and zinc dust. A second layer may be applied as a topcoat coating, such as a powder coating composition based on a polyester resin as a binder and an epoxy group containing component, such as trisglycidylisocyanurate, as a curing agent. Stated applications for this coating are metals such as iron, steel, copper and aluminum, with examples showing use on the outside of a gas tank.
U.S. Pat. No. 7,018,716 discloses a coating comprising an epoxy resin that contains zinc, either as a single coat or as a primer coat, with a topcoat that does not contain zinc, and is reinforced by the addition of fibers and/or by a foaming agent which renders it porous. Stated applications for this coating include high tensile stress steel, such as coil springs.
U.S. Pat. No. 4,804,581 discloses a metal substrate coated with an elastomer-modified epoxy-containing coating primer and a carboxyl-functional material, such as a carboxyl-functional polyester resin, as a top-coat. The coating composition is said to be useful in automotive applications to provide desired anti-chip protection, but the examples show use on grounded steel panels, not highly stressed steel items, such as springs.
For the protection of high tensile strength springs, earlier coating systems used most preferably a combination of a zinc-rich epoxy thermoset primer for exceptional corrosion resistance with an overlying coating of a thermoplastic topcoat applied at a high film thickness to provide superior chip resistance (U.S. Pat. No. 5,981,086). In some cases, epoxy electrocoat was substituted for the zinc-rich primer.
Although typically poorer in chip resistance and cold temperature physical properties, U.S. Pat. No. 7,018,716 reports an epoxy thermoset topcoat with competing performance to the thermoplastic topcoat at reduced cost. Changes in the marketplace with respect to increased demand for zinc metal and associated higher prices have made zinc containing coatings less attractive. The applied cost of zinc containing coatings has also been also hurt by their relatively high density which equates to higher material usage in relation to area coated. Accordingly, there is a need for substantially non-zinc containing coatings for applications such as highly stressed steel.